In general, an electrophotographic image formation apparatus forms a toner image on a transfer belt and transfers the formed toner image to a printing material, thereby forming a permanent image on the printing material. Further, a color image forming apparatus uses a plurality of color toners of different colors one after another to superimpose and form a plurality of toner images. This means that if the rotational speed of the transfer belt when the toner image of a certain color is formed and the rotational speed of the transfer belt when the next toner image is formed do not coincide, the respective images will deviate from each other. This is referred to as so-called “color misalignment”.
Conceivable causes of color misalignment are as follows:
(1) a fluctuation in speed caused by uneven thickness of an intermediate transfer belt;
(2) a fluctuation in speed due to eccentricity of the driving roller that drives the transfer belt; and
(3) a fluctuation in the angular speed of the driving roller.
An example of a method of eliminating cause (1) is disclosed in the specification of Japanese Patent Application Laid-Open No. 10-186787 (Patent Reference 1). This specification proposes a method of forming a registration pattern (a toner image) on a transfer belt, extracting a fluctuation in the traveling speed of the transfer belt based upon pass-by timing of the registration pattern and controlling a driving roller in accordance with extracted fluctuation in traveling speed.
On the other hand, the specification of Japanese Patent Application Laid-Open No. 6-130871 (Patent Reference 2) proposes a method of providing a transfer belt with an optical or magnetic pattern instead of a registration pattern at the time of manufacture, and sensing this pattern by a sensor to thereby detect a fluctuation in the traveling speed of the transfer belt.
With regard to cause (2), the specification of Japanese Patent Application Laid-Open No. 4-172376 (Patent Reference 3) proposes a method of detecting a fluctuation in the traveling speed of a transfer belt by an encoding roller that slides on the transfer belt, and making the distance between the image forming units equal to a whole-number multiple of the circumference of the encoding roller.
With regard to cause (4), the specification of Japanese Patent Application Laid-Open No. 6-175427 (Patent Reference 4) proposes a method of providing the shaft of a driving roller with an encoder and detecting a fluctuation in the angular speed of the driving roll.
In accordance with the prior art described above, only methods of dealing with conceivable specific causes are proposed and it is not possible to deal with color misalignment or inconsistencies in density ascribable to causes other than those conceived. With an image forming apparatus in which cause (1) is dominant, the invention of Patent Reference 1 or 2 is ideal. With an image forming apparatus in which cause (2) or (3) is dominant, however, color misalignment or inconsistencies in density cannot be reduced adequately. Similarly, with the method of Patent Reference 3 or 4 regarding cause (1), color misalignment or inconsistencies in density cannot be reduced adequately. In other words, there is a need for an image forming apparatus in which color misalignment and inconsistencies in density can be reduced even if multiple causes are present.